OTHER STATUS:
Ranked as a "severe threat" by the Tennessee Exotic Pest Plant Council
[71]

Ranked as a "category 1" species (highly invasive) by Vermont
Department of Environmental Conservation [81]

Ranked as a "moderate threat" (species shows invasive behavior,
and known to impact native species, or has a wide distribution and statewide
abundance) by Minnesota Department of Natural Resources [47]

DISTRIBUTION AND OCCURRENCE

GENERAL DISTRIBUTION:
Native to Europe [6,15,26,48,68,73,82,87] and Asia [48,82,87], garlic mustard occurs in
northern Europe from England across Scandinavia to the western areas of the former Soviet
Union, and south to Italy [55]. It was 1st reported in the North America in 1868 on Long Island,
New York [50], and has since established throughout much of the Northeast and Midwest. Garlic
mustard can be found in Canada from southern Ontario east into the St. Lawrence Valley in Quebec
[15,27], as well as around Victoria, British Columbia [15]. In the United States it is
established and invasive in deciduous woodlands and disturbed areas from northern New England west
to eastern North Dakota, and south to eastern Oklahoma and South Carolina [6,14,17,23,27,32,33,48,55,57,60,61,65,68,73,77,82,89]. Occurrences of garlic mustard have also been recorded in Utah,
eastern Colorado, and around Portland, Oregon, Seattle, Washington, and Juneau, Alaska [31,56,75,77,86,87]. The PLANTS database provides a
map of garlic mustard's distribution in the United States.

HABITAT TYPES AND PLANT COMMUNITIES:
In the eastern U.S., garlic mustard occurs in the understory of a variety deciduous forests and
woodlands. It is rarely reported as being associated with conifers, although Cavers and others [15]
state 7 of 37 tree and shrub species found growing over garlic mustard in eastern Canada were
coniferous. While not intended as an exhaustive or definitive list, the following are
specific examples of communities in which garlic mustard has been found.

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

GENERAL BOTANICAL CHARACTERISTICS:
Garlic mustard is an established, cool-season, monocarpic, taprooted, herbaceous biennial
[6,15,26,31,49,61] or occasional winter annual [15,31,61]. The common name is derived from the
scent of garlic, which is noticeably exuded by its aboveground plant parts, especially foliage
[15,31,73,82,86].

Seedlings develop into rosettes 0.8-4 inches (2-10 cm) in diameter during the 1st growing season.
Mature plants produce erect flowering stems up to 4.13 feet (1.25 m) high [15]. Each rosette
usually produces a single flowering stem, although multiple stems from a single rosette occur
occasionally [41]. Flowers are borne in racemes, with fully expanded corollas 0.12-0.48 inches
(3-12 mm) across [6,15,17,26,27,61,68,73,74,82,86]. Average plant biomass is quite variable within
a habitat, between habitats, or between generations within the same habitat, and is strongly
influenced by light. Plants grown under higher irradiance levels typically produce greater biomass
per plant [3].

Seeds are produced in pods (siliques) up to 6 inches (15 cm) in length [15,27,31,74,82]. Fully
developed siliques typically contain 12-19 seeds, and the number of siliques per plant can vary
greatly from 1 to more than 200 [74]. Seeds are oblong to nearly cylindrical [15,61] and about 0.12
inch (3 mm) long [27,31,61].

REGENERATION PROCESSES: Pollination:
Garlic mustard is capable of self-pollinization, as well as cross-fertilization [3,15,17]: both
seem equivalent in effectiveness. Self-pollination often takes place before flowers open [3],
although variation in this ability may exist between populations [3,17]. Cross-pollination has
been observed to occur via generalist insect pollinators, providing a high likelihood of
pollination wherever garlic mustard occurs [3,15,17].

Seed production:
Because a large percentage of flowers typically set fruit, and most ovules develop seeds,
garlic mustard is a prodigious seed producer [17]. Seed production varies between and within
sites and between years, but under shaded, moist (apparently favorable) conditions, dense stands
may produce > 100,000 seeds/m2 [14,15]. Seed production in Ohio ranged from 165 to
868 seeds/plant, depending on habitat and population density [74]. The number of seeds per silique
in a southern Ontario study varied from 6 to 22 with an average of 16. The number of siliques
varied greatly, from 1 or 2 on small plants to up to 150 per plant [15]. Seed production in
several states was:

Seed dispersal:
In forested areas, garlic mustard is typically 1st seen along trails and streams, and can
quickly spread via seeds throughout the forest within a few generations [7]. Seeds generally
fall within a few meters of the plant [50,74], and may be ballistically dispelled from siliques
[49]. Wind dispersal is doubtful. Seeds stick together when damp and adhere readily to small soil
clusters [15]. Seed dispersal rates may accelerate along river corridors [46,50], although there
are conflicting reports regarding the ability of seeds to float [15,74]. Humans may also spread
seeds. Garlic mustard often invades natural areas along roads and trails, purportedly via seed
transport on muddy boots or pant cuffs. Seed dispersal may also be facilitated by roadside mowing,
as well as on mud-encrusted automobile tires [50]. Animals, especially white-tailed deer, may
promote seed dispersal and spread of garlic mustard. Deer are thought to provide an important
seed dispersal vector over short distances by transporting seeds in their fur, although this has
not been tested as of this writing [3,15]. Foraging deer may create microsite disturbances
favorable to garlic mustard dispersal by mixing mineral soil and garlic mustard seeds [49].

Germination:
Seeds of garlic mustard require cold stratification before they can germinate, with 1 season's
overwintering usually sufficient to break dormancy at most North American locations [7]. An
additional year of dormancy was reportedly required prior to germination in southern Ontario [15],
and this lengthier dormancy period may be required in other northern locations [55,70]. Germination
often occurs in early spring and can occur at temperatures approaching 32 degrees Fahrenheit (0 °C)
[7,63]. Low-temperature germination is ecologically important because garlic mustard seedlings
incur a competitive advantage by being the 1st germinants of the season [7,45].

Seed banking:
Garlic mustard produces small but potentially important seed banks. Seed viability has been shown
to drop off substantially after the 1st growing season following stratification, indicating seed
banks of garlic mustard are relatively short lived [7,63]. In a study of garlic mustard seed
biology, roughly 88% of seeds that germinated did so during the 1st spring following production
[7]. In a study comparing garlic mustard populations from contrasting habitats in New Jersey, 3
out of 4 populations were found to maintain a seed bank. The 4th population was located in a
seasonal floodplain where flooding actions were thought to either remove the seedbank or
produce a patchy distribution that was difficult to sample [14].

A small percentage of seeds may remain viable for 4-6 years [7,15,63]. Because
garlic mustard is a prodigious seed producer, elimination of a single season's
crop may not suffice to eradicate the species from an area because germination
and survival of only a few individuals in subsequent years may quickly lead to
repopulation at or near previous levels [7].

Seedling establishment/growth:
Garlic mustard seedlings emerge in early spring, just before or simultaneous with germination
of native spring ephemerals [49]. They establish during periods of relatively high light
availability in the forest understory prior to canopy leaf-out, typically with reduced
interspecific competition and drought potential [7,15,45]. Greatest mortality rates occur in
spring during the seedling stage [15]. Seedling mortality can vary substantially, often depending
on moisture availability [14]. Initial seedling density may be very high (20,000 seedlings/m
2) [49,74]. In reports where natural spring seedling densities were approximately
3,100 to 5,600/m2, only about 1% to 16% survived to produce flowers the following year
[14,15]. Two consecutive cohorts retained similar numbers of mature flowering plants during their
2nd spring, despite having initial seedling densities differing by more than 100% [3].

SITE CHARACTERISTICS:
Garlic mustard has a wide tolerance of environmental conditions for growth and reproduction,
including moisture regimes ranging from periodically flooded areas to dry sand forest [15,42],
light environments ranging from open fields to shaded forest interior [12,14], and a range of
various soil characteristics including texture [14,15,57], nutrient level [14], organic matter
content [14,15], and pH [4,14]. It is apparently not found on acid soils in Indiana, Kentucky,
Massachusetts, or the Canadian Shield region [15], and is absent from undrained peat and muck
soils [49].

Garlic mustard may be less competitive in areas with low soil pH, as evidenced by an experiment
demonstrating a significant positive correlation (r = 0.98; p < 0.001) between plant dry weight
and soil pH. This has been hypothesized as a contributing factor in the limited colonization
of garlic mustard in the southern third of Illinois, where soils are more acidic than in the more
heavily colonized central and northern sections of the state [4]. Inhibition of garlic mustard by
acidic soils may explain its apparent absence from conifer-dominated communities [66].

Garlic mustard appears to favor shaded sites [50], and is often found in dense groups of
nearly pure stands, sometimes covering large areas, particularly under moist shaded conditions
such as mature eastern deciduous woodlands. In drier or more open areas plants increase allocation
to fruit production, perhaps in response to observed declines in seed weight, seed germination,
and seedling survivorship [14,46]. While biomass production may be greatest under full sun [15],
and garlic mustard plants can also be found under dense shade, they are most commonly found in
woodland understories with partial shade and are probably less invasive under extreme conditions
of light or shade [49]. Nuzzo [50] describes typical habitat in Illinois as mesic upland or
floodplain forest, usually shaded, and often associated with some type of disturbance. Despite
its apparent affinity for moist shaded environments, garlic mustard is not tolerant of growing
season inundation, which may limit its ability to invade wetland communities [49].

Most populations of garlic mustard appear to be connected to some form of disturbance [14,49].
Garlic mustard is often associated with anthropogenic disturbance such as trails, roads, or
railroads [49,50], and less commonly, in farm fields and gardens [50]. Garlic mustard is
sometimes linked to naturally disturbed habitats such as floodplains and riverbanks, where the
combination of flooding as a dispersal agent and moist, shaded conditions may promote invasion [46].
Garlic mustard was invasive in relatively undisturbed woodlands in central Illinois. Establishment
was thought to occur where small-scale anthropogenic and natural disturbance removed competing
vegetation, such as areas browsed by white-tailed deer [3].

Experiments examining mechanisms that
link disturbance and garlic mustard occurrence and spread are scarce. One study showed that
disturbance of soil in a young hardwood forest in northern Kentucky resulted in lowered garlic
mustard densities compared to undisturbed plots [39]. An experiment in a southwestern Ohio
deciduous forest examined the effects of small-scale litter disturbance on garlic mustard
invasiveness. There were no differences (p = 0.7184) in garlic mustard germination, rosette
survival, growth, or reproduction among total litter removal, partial litter removal, and control
treatments, indicating forest floor disturbance alone may not be a prerequisite for invasion [46].

More research is needed to help understand factors that influence garlic mustard
invasiveness and habitat invasibility, particularly for the role of disturbance. In particular,
questions involving which life history traits are affected by disturbance seem most appropriate.
Experiments that separate disturbance-mediated dispersal from other interactions between
disturbance and garlic mustard invasiveness might provide important insights leading to more
effective management prescriptions.

SUCCESSIONAL STATUS:
Garlic mustard occurs in communities that represent a wide range of successional stages, from
prairie openings to understories of mature, shade-tolerant eastern hardwood forests. While garlic
mustard colonizes a variety of sites, it is often mentioned with particular concern to
invasiveness in the herb layer of mature eastern deciduous forests, since these communities were
thought to be somewhat resistant to invasion by nonindigenous plants (see
Impacts and Control). In some
areas of eastern deciduous forest, dense garlic mustard stands may inhibit recruitment of woody
seedlings, perhaps altering successional trajectories [45].

The ability of garlic mustard to invade and compete in habitats with light environments ranging
from partial to deep shade may be due to its ability to acclimate to variation in irradiance
[2,15]. Despite substantial plasticity in photosynthetic response to variation in irradiance,
photosynthetic rates of garlic mustard at high light levels lag behind those of species typically
found in unshaded environments, inhibiting the competitiveness of garlic mustard under these
conditions [20]. Nevertheless, the ability of the species to acclimate to a wide range of light
environments almost certainly contributes to its ubiquitous and invasive nature [2].

Garlic mustard is often mentioned in association with oak savannah communities which, when
viewed from the context of fire as the determinant of successional trajectory, represent a
transitional state between grassland and forest. For example, garlic mustard was present mainly
in areas of lower ambient light levels in a northern Illinois oak savanna remnant, invading
where reduced fire frequency resulted in increased tree canopy cover [11]. Because the
presence of garlic mustard may inhibit the ability of a forest understory to carry surface
fire [49], invasion of garlic mustard could potentially accelerate succession in these oak
savannas by further suppressing fire.

SEASONAL DEVELOPMENT:
Garlic mustard seedlings almost always emerge in early spring. In shaded areas growth is rapid
until deciduous trees form leaves, after which growth slows. Garlic mustard growing in more open
areas may continue steady production of new leaves into summer [74]. Germination usually
occurs in February or March, depending on conditions and location [3,15,63,74]. There are
reports of germination being triggered by the 1st warm rains of the season [49].

Seedlings develop into rosettes early during the 1st growing season. In central Illinois,
garlic mustard leaf area increased into early June, then declined slightly or remained nearly
constant until the following spring [3]. Rosettes overwinter and may retain considerable green
leaf tissue. They are capable of winter growth during periods of above-freezing temperatures
and no snow cover [15,74], although some leaf tissue may be destroyed by extended periods of
subfreezing temperatures [3]. Anderson and others [3] observed that new leaf growth was initiated
when temperatures warmed to greater than 37-41 degrees Fahrenheit (3-5 °C) for several hours per
day. Substantial winter mortality is possible. Nearly 80% mortality was recorded from November
1989 to May 1990 in a northern Illinois dry-mesic upland forest [51]. Fall and winter growth
varies by season and location, but by late winter rosettes have leaf primordia surrounding 1
or more terminal flowerbuds [74].

Garlic mustard plants that survive winter undergo rapid bolt growth early in the 2nd spring
(March in many areas) [14,15,74]. Reinitiation of growth during this period includes increases
in leaf production and elongation of flowering stems, and all surviving plants bolt [15,74].
In central Illinois, plants bolted in March, growing quickly to a maximum
leaf area by late April or early May. Stem growth was also rapid during this
period, elongating at an average rate of 0.76 inch/day (1.9 cm/day) between April 18 and May 13.
Following this period of rapid growth, leaf area quickly declined. Nearly
all 2-year-old plants were devoid of green leaves by early July [3]. The fact
that bolting occurs very early in spring may contribute to garlic mustard's
competitive ability by limiting light and space resources that might otherwise
be available to native herbs [45].

Flowering occurs in spring of the 2nd season, usually from early April to early June,
depending upon location and season [3,15]. In central Illinois lower flowerbuds appeared in
early April, and reached the maximum number per plant about 2 weeks later. Flowering,
and the presence of green fruit, peaked around mid-May. Fruit dehiscence began in late May or
early June, and by late June no green fruits remained [3]. Damage to terminal flowerbuds can
initiate flower production via axillary buds, sometimes occurring well into summer, but these
late-season blooms rarely produce viable fruit [3,74].

Typically, seeds are ripe and begin to shed by mid-June and continue to drop throughout summer
and into autumn. Seeds have mostly all dropped by early November [3,15]. Once mature plants
have produced fruit they senesce and die [74]. Some variability exists regarding the timing of
senescence, which is probably related to environmental conditions such as irradiance levels or
moisture availability [3].

FIRE ECOLOGY

FIRE ECOLOGY OR ADAPTATIONS:Fire Adaptations:
Although garlic mustard plants are readily top-killed when exposed to fire,
they may ultimately survive by sprouting from the root crown [54]. Ecological conditions that
permit sprouting are not well understood and it is unclear to what extent resprouted plants are
capable of producing viable seed (see Fire
Effects section of this summary).

At the population level, garlic mustard may be adapted to perpetuate itself in
mixed-severity or low-severity surface fire regimes, although this has not been
quantified. Even though individual plants may be killed by fire, postfire
conditions may be favorable for rapid population expansion because of increases
in the area of disturbed habitat and, depending on the extant community,
temporary reductions in interspecific competition. Additionally, garlic mustard
seed banks may facilitate rapid recolonization of disturbed areas [14]. For example,
3 consecutive years of prescribed burning in a central Illinois black oak forest,
which were described as "hot and fast" with flame lengths to
4 feet (1.2 m), failed to eradicate garlic mustard populations. This was attributable, in
part, to the protection afforded a small number of plants by refugia such
as the lee of a downed log or an area of damp litter [54]. The
ability of individual plants to escape mortality will depend upon fire
severity and the heterogeneity of the fire landscape.

Fuels: Although it has been demonstrated that
fire can top-kill garlic mustard (see
Immediate Fire Effect on Plant),
it is also apparent that garlic mustard plants can be difficult to ignite. Nuzzo [49]
noted that low fuel loads, coupled with abundant green garlic mustard plants, "which on
occasion literally extinguished fires", made it difficult to achieve prescribed
fire objectives.

Fire Regimes:
Garlic mustard may be found within understory surface, stand-replacement, mixed-severity fire,
and nonfire regimes [13]. Because garlic mustard has become
established only relatively recently in most areas in North America, and because natural
fire regimes have been substantially altered in many of these areas, predicting
the response of garlic mustard to any particular fire regime is speculative.
In some areas colonized by garlic mustard, estimated mean fire return intervals
may be longer than the time in which garlic mustard has been present. As natural areas and
preserve managers reintroduce fire into locations where natural and anthropogenic fire has
been suppressed in recent times, the response of this and many other species may become better
understood. Those who intend to reintroduce fire where it has been absent for a
substantial period are encouraged to plan and implement research and monitoring programs and
share their findings.

Fire regimes of some of the plant communities in which garlic mustard occurs are summarized below.
For further information, see the FEIS summaries of the dominant species listed
below.

FIRE EFFECTS

IMMEDIATE FIRE EFFECT ON PLANT:
Garlic mustard is often top-killed when exposed to fire. A prescribed burn in the understory
of a northern Illinois hardwood forest apparently removed all aboveground garlic mustard biomass [30].
Prescribed burns in a central Illinois black oak forest conducted both in the fall
and in mid-spring removed nearly all garlic mustard rosettes [54].
Although there was no immediate postfire survey of plants mentioned in the
article, Luken and Shea [41] suggest garlic mustard "plants are readily killed
by mid-intensity dormant season fires". Emergent seedlings may also be killed by
fire [54].

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
It has been suggested that dense stands of garlic mustard may be able to resist low-severity
fire, such that "abundant green garlic mustard plants...may literally extinguish
fires" [49], but detailed descriptions of the direct effects of fire on garlic
mustard plants (or vice versa) are scarce. Such observations may be confounded
by the inherently patchy nature of mixed-severity fire regimes in many eastern
deciduous forests where garlic mustard may commonly be found. For more information see the
Fire Ecology section of
this summary.

PLANT RESPONSE TO FIRE:
Garlic mustard has at least some ability to sprout from the root crown following damage
by fire. By excavating charred rosettes, Nuzzo and others [54] found that adult plants resprouted
from adventitious buds on the root crown located just below the soil surface following a mid-spring
burn. In a northern Illinois oak woodland, garlic mustard reportedly resprouted several weeks
following complete top removal by a prescribed fire conducted in late March [30]. Repeated fall
burning (2-3 annual burns) did not reduce abundance or relative importance of garlic mustard in an
eastern mesophytic forest understory in Kentucky [41].

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
There is some indication that garlic mustard is capable of sprouting following
fire, but several questions remain. To what extent is postfire sprouting in
garlic mustard influenced by fire severity? What, if any, physiological
conditions promote or constrain postfire root crown sprouting? To what extent are
resprouting plants successful at producing seed?

Nuzzo and others [54] reported that a fall burn in a central Illinois black oak forest
removed 79% of the litter layer, and very few adult garlic mustard plants were encountered in
these plots the following spring. Conversely, many garlic mustard plants resprouted following
a mid-spring burn at the same site that resulted in removal of only 32% of the litter layer.
Spring burn plots retained a damp 0.4- to 0.8-inch (1-2 cm) layer of litter which
seems to have protected the root crowns of top-killed plants, fostering survival via sprouting
of multiple secondary shoots from adventitious buds located just below the soil surface [54].

Hintz [30] conducted a late-March prescribed burn in a mesic upland oak-hickory
forest in northern Illinois. Garlic mustard established following the fire,
although it is unclear whether these were sprouting burned plants or new spring
seedlings. The burn was conducted near the time when seedling emergence might be expected,
leaving some question as to which life-cycle stage was observed to be "sprouting".
There is reference to "very little" garlic mustard producing seed that summer,
intimating that at least some adult plants were present both prior to and after the fire.

Luken and Shea [41] conducted a prescribed fire experiment in a northern
Kentucky mesic deciduous forest in which they showed that garlic mustard plants
could be removed by a fall burn. Yet it was also apparent from this experiment that populations
can persist following even repeated burns. Garlic mustard remained the dominant species in the
herb layer of both burned and unburned plots through 3 seasons of fall burning,
and beyond. The authors proposed 3 possible explanations. First, persistence of individual garlic
mustard plants immediately following fire may result from the patchy nature of many understory
or mixed-severity burns. Under such conditions some extant plants may escape damage, and because
of its ability to self-pollinate [3,15,17], the survival of a single plant may be sufficient to
perpetuate a population. Second, the data of Luken and Shea [41] showed that burning resulted in
higher densities of flowering stems compared with control plots. They speculated this as being
due to either resprouting or release from competition. No observations of sprouting were reported.
Third, even if all plants are killed, the existing seed bank may remain viable for several years
[7,14], requiring subsequent annual burns to completely eradicate the population.

The Research Paper by Bowles and others 2007 provides information on postfire responses of several plant species, including garlic mustard, that was not available when this species review was originally written.

FIRE MANAGEMENT CONSIDERATIONS:
Control of invasive garlic mustard populations using prescribed fire, especially as a single
management tool, appears to be difficult. Some temporary control is likely, but difficulties
sustaining long-term control are confounded by a) the patchiness of understory
and mixed-severity fires, b) the biennial nature of the species, c) the moderately persistent
seed bank, and d) garlic mustard's propensity for rapid population increase
(see Regeneration
Processes) [41,54,67].

It may be possible to substantially diminish the number of individuals in a
garlic mustard population with repeated burn treatments. But
prescribed burning, especially during the growing season, could actually increase the
relative importance of garlic mustard [3,41,54]. A prescribed burn conducted in May in a
northern Illinois dry-mesic upland deciduous forest effectively reduced cover of garlic mustard,
from a pre-burn 29.4% cover to 2.3% cover, postfire year 1. But May burning also damaged
the native forb community, where total stem density of major herbs and small shrubs was
reduced by 32% and average number of species per plot was reduced by 35%,
postfire year 1. Although native plants subsequently showed gradual recovery,
these effects were detectable for 3 years, most notably for Jack-in-the-pulpit
and stickywilly. Garlic mustard recovery was more rapid. Within three years
following burning garlic mustard had rebounded to 17.3% cover compared with a
pre-burn level of 29.4% [67].

Dormant-season burns, while less likely to have
negative effects on indigenous flora, also appear to be less effective at
killing garlic mustard rosettes. After 3 years following a March
prescribed burn at the above location, both garlic mustard and native herb cover
had returned to approximate pre-burn levels [67].

It has been suggested that a narrow window of time exists during early spring in some areas
and in some years, during which garlic mustard may be more effectively controlled by fire
without damaging native plants. This hypothesis remains untested as of this writing [67].
Also, spring burns may increase seedling survival. Fires of insufficient severity may spare a
sizable fraction of seedlings protected by the unburned portion of the litter layer. Additionally,
a spring burn timed too early may permit survival of garlic mustard seedlings that germinate
after treatment. In addition to greater initial seedling survival, removal of a portion of the
litter layer may also provide a more favorable environment for growth and development of
garlic mustard rosettes [54].

Apparently not all fires are equally effective at top-killing garlic mustard. The effectiveness
of prescribed spring and fall burn treatments in reducing garlic mustard populations in an oak
-dominated dry-mesic upland forest in northern Illinois was directly related to fire
"intensity". "Low-intensity" burns, with flame lengths up to 1.2 inches (3 cm),
were patchy and frequently extinguished within plots. These "low intensity" burns had
little to no effect on garlic mustard plants, whether seedlings or adults, regardless of season
of burning. It was suggested that abundant green garlic mustard plants frequently extinguished the
"low intensity" fires. "Mid-intensity" burns, with flame lengths up to 3
inches (15 cm), burned through most of the plots and significantly reduced the presence of
garlic mustard. Adult plant densities were reduced by both spring and fall burns, as well as
repeated fires, although single spring burns were most effective [49].

In areas with long fire-return intervals where favorable conditions for conducting effective
prescribed burns may be rare to nonexistent, especially repeated annual burns, or where
fire-sensitive native species exist, prescribed fire may be unsuitable as a management tool.
Nevertheless, in areas with a fire-tolerant native flora, frequent prescribed burning may deter
garlic mustard invasion by both directly killing invading plants, and perhaps in some areas by
enhancing growth of native herbaceous competitors and thereby reducing habitat for garlic mustard
colonization [49,88]. It is highly likely that managers who use fire to control garlic mustard may
need to augment burn treatments with 1 or more additional control methods, such as
pulling or herbicide use to achieve acceptable levels of control (see Impacts
And Control for more information on other control methods).

MANAGEMENT CONSIDERATIONS

IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Use of garlic mustard as a forage species by white-tailed deer is unclear [15,56]. White-tailed
deer may avoid grazing garlic mustard in favor of native herbaceous plants, although this has not
been empirically tested [3,49,56]. It is likely that white-tailed deer graze a variety of
understory herb species in areas typically susceptible to garlic mustard invasion, and can have a
dramatic negative impact on some native herb populations [1]. Deer grazing of native herbaceous
plants may enhance garlic mustard at the expense of native species by providing small-scale soil
disturbance and by reducing interspecific competition. White-tailed deer may provide small-scale
disturbances suitable for garlic mustard colonization within forested areas by trampling and
exposing soil. In addition, selective herbivory may enhance garlic mustard at the expense
of the preferred native species [3,49,56].

Garlic mustard may be deleterious to some species of butterfly. Adults of several butterfly
species lay eggs on garlic mustard instead of their native plant hosts.
Because larval development on garlic mustard is often fatally inhibited, this
can result in garlic mustard acting as a population sink for these butterfly
species, a particularly perilous problem for rare species such as the West
Virginia white butterfly (Pieris virginiensis) [10,56,59].

PALATABILITY:
Garlic mustard is apparently palatable to livestock. It is thought to taint the flavor of
milk in dairy cattle [15].

OTHER USES:
Garlic mustard was historically eaten as a green vegetable by Europeans and is high in
vitamins A and C. It was also used as a medicinal herb in Europe [15].

IMPACTS AND CONTROL:Impacts:
The control of garlic mustard may be desirable to undisturbed deciduous forests of the eastern
and midwestern United States and southern Ontario [3,15,17,49,55]. In forested natural
areas, garlic mustard has the potential to dominate the herb layer [41,52,56,91]. Invasion of
mature eastern deciduous forests by garlic mustard is notable because these habitats were thought
to be relatively resistant to nonindigenous plant invasion, particularly by herbaceous species
[43,45,55,56]. From the results of a greenhouse study examining the competitive potential of
garlic mustard, Meekins and McCarthy [45] postulated that competition for light within dense
garlic mustard stands might inhibit oak regeneration in the understory of eastern deciduous
woodlands. However, this same study failed to show greater levels of interspecific competition
among garlic mustard, jewelweed, and box elder, 2 potential understory associates.

McCarthy [43]
demonstrated removal of garlic mustard from a deciduous forest understory resulted in increased
richness and abundance of understory species, especially annuals and woody perennials. Garlic
mustard may be particularly detrimental to native spring ephemerals in eastern deciduous forest
understories [15]. McCarthy [43] failed to demonstrate a link between the magnitude of garlic
mustard infestation and native species diversity. Removal experiments, while providing some
insight into possible effects of nonindigenous plant invaders, may be inherently limited in
their ability to reflect impacts of invasives on preinvasion communities [84]. Limited
and conflicting evidence surrounding the assumption that garlic mustard infestation necessarily
results in reduced richness and cover of native herbaceous species points out the critical need for
more research in this area.

The allelopathic potential of garlic mustard has received some study, with
mixed results. McCarthy and Hanson [44] found little evidence of allelopathic
effects of garlic mustard on several plant species studied. They attributed
the success of garlic mustard invasiveness strictly to its competitive
abilities. Other evidence indicates at least the possibility for allelopathic interference
between garlic mustard and neighboring herbaceous plants, as well as the possibility for
toxicity against mycorrhizal fungi [35,80]. Roberts and Anderson [64] found a significant
negative correlation (r2 = 0.29; P< 0.05) between garlic mustard density
in the field and the mycorrhizal inoculum potential of the soil. McCarthy [43] found garlic
mustard inhibited establishment of seedlings of other species, yet no quantitative relationship
could be discerned between garlic mustard biomass and native species diversity.
This finding suggests that the mere presence of garlic mustard depresses native diversity,
perhaps an allelopathic effect. Further research is needed to a) determine what mechanisms, if
any, are responsible for garlic mustard allelopathy, and b) sort out the relative
effects of allelopathy vs. resource competition in interactions between garlic
mustard and native plants.

Control:
The biology of garlic mustard presents significant challenges to its control
because it simultaneously possesses characteristics of native forest herbs such
as shade tolerance and relatively large seeds, as well as characteristics often
ascribed to weeds such as xenogamy and autogamy, and high seed production and germination
under a range of environmental conditions. It is also not impacted by its native herbivores
and parasites [3,5,17,44]. While garlic mustard invades relatively undisturbed woodlands,
invasion may be expedited by natural and anthropogenic disturbance that removes
competing native vegetation. Once garlic mustard becomes established, further dispersal and
perpetuation within a particular habitat may require little to no further disturbance [46,55].

Deciduous forest fragments that are isolated in an otherwise predominantly agricultural landscape
may be more resistant to garlic mustard invasion, due to limited seed sources and inhibitive
dispersal distances [12]. However, in areas with large populations of white-tailed deer, even
these insular forest remnants may become colonized by garlic mustard.

As with most invasive plants, deterrence is the
most effective strategy against garlic mustard. This includes annual monitoring
and removal of all invading plants prior to seed production. Garlic mustard is
prolific partly because of its ability to self-pollinate. A single individual
can produce large numbers of genetically
similar but interfertile progeny, which in turn may colonize even small, local
microsite disturbances, leading to a potential garlic mustard outbreak. Allaying invasion may
require reducing habitat perturbation in susceptible areas and promoting
the health of native plant communities [3].

Garlic mustard population densities may oscillate widely from
year to year [56]. Its biennial nature and its seed banking propensity can lead
to occasions in which dense stands of garlic mustard appear where none were
apparent the year before, and then seemingly disappear the following year only
to reappear yet again in subsequent seasons. Further, in years where rosettes
are apparently sparse and may evade detection, those monitoring such sites may
easily but falsely conclude that garlic mustard is absent. In previously
infested areas or areas of suspected susceptibility, careful annual monitoring
may be the only way to ensure that garlic mustard is indeed absent from the site.

Once garlic mustard appears within an area,
management activities should focus on preventing seed production. While
most seeds of garlic mustard tend to germinate during the 1st or 2nd spring
following their production, a small number of seeds remain
within the seed bank and may germinate over the next several years. Because garlic mustard
seed banks may remain viable for up to 6 years, long-term control for a particular stand
requires vigilant attention for several consecutive seasons [3,7,14,49].
Even after successful management leads to the apparent absence of garlic
mustard, continued periodic monitoring is prudent. A method for destroying
seeds of garlic mustard in the soil that would not harm seeds of other species
has not been determined [7].

Because of the biennial life-history strategy of garlic mustard, eradication treatments conducted
during spring, after seedlings have germinated and before adults can produce viable seed, have
the advantage of affecting 2 generations simultaneously [49]. Ideally, this
maximizes the kill of new germinants and seedlings, as well as prevents seed
production in adults. Since natural mortality is greatest at the seedling stage garlic mustard
may be most vulnerable to control efforts during this time [20]. One potential downside to this
strategy is that delaying treatment too late into spring risks unwanted effects on native
spring emergents.

An alternative approach is to delay management activities until after the 1st growing season
to take advantage of significant natural mortality of rosettes. First year garlic
mustard mortality at a site in northern Illinois was estimated at greater than 95%
between April and November [51]. This strategy may be especially
prudent when the control method requires intensive labor, such as cutting or
hand-pulling plants, if minimizing quantities of applied chemicals is desired,
or simply if costs of more intensive management activities are prohibitive.

Control of garlic mustard has been tested using several different methods. Since
a single control method is rarely 100% effective, a combination of more than 1 may often
be a useful strategy. Regardless of methodology, treatments for eradication of garlic mustard
must be applied over the entire area of infestation to prevent seed production.

Manual or Mechanical Removal: Pulling entire plants may be an effective
method for control of garlic mustard. Care should
be taken to remove as much of the root system as possible, to reduce resprouting
potential. Pulling can cause soil disturbance and redistribute
seeds stored within the upper soil horizons. This problem may be mitigated by
thoroughly tamping disturbed soil after pulling. Generally speaking, cutting
results in fewer disturbances than pulling. However, pulling may be done at any
time during the plant lifecycle, while cutting must be performed during the 2nd growing season
while the flowering stem is elongating. Due to the labor-intensive nature of cutting and
pulling plants, these practices may only be practical in small or lightly infested areas,
especially where burning or herbicide application is inadvisable [49,56].
Hand removal may be most useful for preventing establishment of new garlic
mustard colonies in previously uninfested areas [43].

Control may be accomplished by cutting flowering stems, i.e. using sickles,
clippers, or string trimmers, prior to seed production and dissemination. Cutting as close to
ground level as possible appears to be most effective. Nuzzo [49] found that
cutting at ground level killed 99% of plants and resulted in virtually no seed
production, while cutting at 4 inches (10 cm) resulted in 71% mortality and 98% lower total
seed production. Mortality was 6% in control plants during the 3-month study period. Cutting
plants prior to full flowering or the onset of seed development may result in production of
additional flowering stems from buds located on the root crown [56]. However, waiting until
after plants have finished flowering risks dissemination of viable seed. Cut or pulled plant
material should consequently be removed from the site and destroyed whenever possible to
minimize the risk of inadvertently distributing viable seed [56,70].

Mowing may be similar in effect to cutting, but with more possible negative consequences.
Mowing of flowering plants may result in regrowth of new flowering shoots, although this
response reportedly diminishes as the season progresses [15]. While mowing may be convenient
in large, relatively open areas of infestation such as roadsides, this practice may be more
problematic than cutting, as described above. Mowing may promote seed dispersal and is more
likely to be indiscriminate regarding which plant species are destroyed. Mowing equipment may
also create more disturbed habitat that is likely to be recolonized by garlic mustard [56].

Prescribed Fire: In areas with a fire-tolerant native flora, frequent prescribed burning
may deter garlic mustard invasion by both directly killing invading plants, and perhaps in
some areas by enhancing growth of native herbaceous competitors and thereby reducing habitat
for garlic mustard colonization [49,88]. For more information about
using prescribed fire as a management tool to control garlic mustard, see
the Fire Management Considerations section of
this summary.

Chemical Control: Chemical control of invasive plants such as garlic mustard
can be effective, particularly against large areas of infestation or dense
monotypic colonies, and especially when considered within the context of an
integrated management plan [47,49]. This report briefly examines the
effectiveness of selected chemicals for controlling garlic mustard, some issues
involved in the timing of application, and potential effects on native plant
communities. Use of herbicides in natural areas should be cautiously considered,
and appropriate education and training should be sought before proceeding.
Particular caution should be exercised with the use of Bentazon or Acifluorfen.
Bentazon is very soluble in water and does not bind to soil well, leading to
potential groundwater contamination problems. Acifluorfen is toxic to fish, is
moderately persistent in soil and kills native grasses and herbs, and can cause
serious eye injury [79]. For further information regarding the use of herbicides
in natural areas for control of this and other invasive plant species, see
the Weed Control Methods Handbook
[76].

The effectiveness of 2,4-D against garlic mustard is questionable [56]. Use of 2,4-D in mixtures
with other chemicals may improve its effectiveness, but scant evidence is available [15,56].

Application of 1% and 2% glyphosate during the dormant season significantly
(p < 0.05) reduced adult garlic mustard cover and density in mesic
and dry-mesic upland forest and mesic floodplain forest in northern Illinois,
but also damaged other species that were green at the time, especially sedges
and white avens [53]. Treatment with foliar-applied glyphosate also significantly (p < 0.05)
reduced adult densities of garlic mustard, regardless of spring
or fall application, in a northern Illinois oak woodland. Seedling frequency in
these same plots was significantly (p < 0.001) reduced by spring application [49].

Dormant-season application of bentazon was less effective at controlling
garlic mustard in northern Illinois mesic deciduous forest, but showed none of the
nontarget kill associated with glyphosate. At these same sites, application of acifluorfen
during dormant season was highly effective againstgarlic mustard, but also killed most
native herbaceous vegetation, which was mainly dormant at the time of application[53].

Use of systemic, nonselective herbicides during the growing season may not be practical
in some areas due to deleterious effects on native ground-layer competitors. In
these cases, dormant season application may be preferable in order to maintain viable
populations of native competitors [49]. Nuzzo [49] found no
difference in effect between single herbicide application and twice applied
treatment to the same generation of plants (spring and fall of the same
year, fall and the following spring, or 2 consecutive springs). It was
suggested that management efforts focus on single applications to successive
generations of plants. Fall herbicide application may be a prudent option when risk of
negatively affecting native spring-emergent herbs exists. Higher garlic
mustard rosette densities in fall may require higher volumes of applied herbicide to be
effective [51].

Mid-summer application of bentazon reduced garlic mustard cover by 94-96% in previously
dense stands of garlic mustard rosettes in northern Illinois. Similar applications of acifluoren
were less effective, but still significantly reduced garlic mustard cover by
30-46%. Mortality in control plots over the same period was 15%, and
not statistically significant. Chemical control activities conducted during the
growing season, as above, might be justified when target species densities
overwhelm the native flora [52].

Biological Control: Biological control methods for garlic mustard are not yet developed, but
investigations are under way. Several insects that are associated with garlic mustard in its native
European habitats are being tested to examine their potential effectiveness as
control agents [56]. Fungal pathogens may also have some potential use
against garlic mustard. For instance, garlic mustard has
shown susceptibility to a fusarium root rot (Fusarium solani) [16].

18. DeLoach, C. Jack. 1997. Biological control of weeds in the United States and Canada. In: Luken, James O.; Thieret, John W., eds. Assessment and management of plant invasions. New York: Springer-Verlag: 172-194. [38164]

32. Kartesz, John T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. 1st ed. In: Kartesz, John T.; Meacham, Christopher A. Synthesis of the North American flora (Windows Version 1.0), [CD-ROM]. Chapel Hill, NC: North Carolina Botanical Garden (Producer). In cooperation with: The Nature Conservancy; U.S. Department of Agriculture, Natural Resources Conservation Service; U.S. Department of the Interior, Fish and Wildlife Service. [36715]